1. Field of the Invention
This invention relates to the field of gyroscopes and rotation rate sensors, and more particularly to rotation rate sensors that utilize an oscillating mechanical element to sense rotation rate.
2. Description of the Related Art
Many methods have been developed to sense rotation rate. Gimbaled spinning wheel, ring laser, and vibrating ring gyroscopes have all been successfully developed for this purpose. Many applications exist for devices that can sense rotation, such as traction control and ride stabilization systems for cars, and stabilization systems for aircraft and spacecraft. However, existing gyroscope designs tend to be too expensive for mass market applications.
One method of sensing rotation rate utilizes a vibrating mechanical element, in which the Coriolis force that arises when the element is rotated is used to measure rotation rate. Devices of this type include the hemispherical resonator gyroscope and the vibrating ring gyroscope. Such vibratory gyroscopes have no rotating parts and are easily miniaturized using micro-machining techniques. These type of devices are described in M. W. Putty and K. Najafi, "A Micromachined Vibrating Ring Gyroscope," Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., June, 1994. However, these gyroscopes tend to be either very expensive or low accuracy devices.
A mechanical element is fixed at one end and extends along a y-axis. The free end of the element is made to oscillate along an x-axis. The element is then subjected to rotation about the y-axis. A Coriolis force, induced by the rotation of the element and orthogonal to the axes of oscillation and rotation, i.e. along the z-axis, will cause the path described by the element to change, unless constrained by a counteracting force. Rather than continuing to oscillate back and forth along the x-axis only, under a Coriolis force acting alone, i.e. with no counteracting forces, the element would describe an elliptical pattern, with motion in both z and x axes. The Coriolis effect is a well known phenomenon and is discussed in D. Considine, Van Nostrand's Scientific Encyclopedia, Seventh Edition, Van Nostrand Reinhold (1989), page 773. The Coriolis effect will cause the element, oscillating laterally along the x-axis and rotating about the y-axis, to experience a Coriolis force F.sub.c along the z-axis as follows: EQU F.sub.c =2mW.sub.r xV.sub.1
where m is the mass of the element, W.sub.r is the element's rotation rate and V.sub.1 is the lateral velocity of the element. If a force is applied to the element to counteract the Coriolis force and prevent z-axis displacement, measuring this counteracting force will give the Coriolis force, which is directly proportional to the rate of rotation.
A rotation rate sensor based on this principle having a properly designed mechanical element, an accurate sensor for measuring z-axis displacement, and circuitry to control lateral oscillation and to generate a force that prevents displacement along the z-axis, can overcome the accuracy and cost deficiencies of other vibrating element rotation rate sensors.